1. Field of the Invention
The invention relates to electrically controllable panel display devices employing positive dielectric anisotropy liquid crystalline materials as electrically active media and more particularly relates to such display devices in which the size, shape, and location of two-dimensional display patterns can be changed in discrete steps.
2. Description of the Prior Art
Nematic liquid crystal materials offer utility in electric current controlled display devices of the flat panel type. One prior art application of such electrically controllable dynamic scattering materials employs a structure which is a cell generally of sandwich configuration comprising first and second sets of transparent planar electrode systems and a specularly reflective mirror spaced from the second electrode system. Between the two electrode systems is located a relatively thick layer of active nematic material. With no electric field applied between the two electrode sets, the liquid crystal material is optically transparent. Thus, if the mirror reflects a black background, the cell looks black to a viewer looking into it through its transparent first electrode system. When an electric field is applied between the two spaced electrode systems, the liquid demonstrates turbulence and abruptly loses its transparent characteristic, scattering any light flowing into it through its transparent first electrode system. In this state, the scattered light is returned to the viewer, and the apparent color of the cell generally has the same spectral content as the light passing into it through the first electrode system. When the electric field is removed, the material abruptly reverts to its transparent state and looks black to the observer.
Such prior art displays have made advantageous use of the properties of liquid crystal compositions. However, the displays require electrode systems on interior surfaces of both the first and the second wall elements of the cell, which electrode systems accordingly require careful alignment during assembly and the displays are therefore expensive to produce. Such prior art devices, having conducting electrodes on each side of the nematic layer, are also subject to accidental short circuiting between electrode systems. A factor further degrading the life of the prior art displays is that the primary phenomenon producing the display is induced by electric current flow, rather than by effects directly responsive to the electric control field. Such substantial current flow tends to cause the liquid crystal materials to deteriorate, reducing the life span of the display. Relatively high operating power is also required. Prior art displays of this type do not permit wide angle viewing and suffer from parallax problems and from unwanted specular reflections in the reflective mode.